The present invention is generally directed to toner compositions, and more specifically to encapsulated toner compositions and processes thereof. In one embodiment, the present invention is related to color encapsulated toner compositions comprised of core components such as, for example, polymer resins and colorants; thereover a polymeric shell or shells overcoated with an outer polymer layer. Another embodiment of the present invention relates to processes of preparing encapsulated toners comprised of a core containing a polymer resin, or plurality of polymer resins and colorants, such as black, red, green, blue, cyan, yellow, magenta pigments or dyes, and mixtures thereof, encapsulated within a shell comprised of certain polymeric surfactants, such as a cellulose surfactant, and an optional shell comprised of condensation polymer such as polyurea, polyurethane, polyester, polyamide, mixtures thereof, or the like, formed by interfacial polymerization, and overcoated thereon a polymeric overcoating formed by free radical polymerization.
One of the primary functions of the polymeric surfactant shell and the condensation polymer shell is to provide mechanical strength to the toner, while that of the overcoating layer is to reinforce the shell in achieving effective containment of core components, particularly the colorants. Effective containment of core components would enable complete, or substantially complete nullification or passivation of the charging effects of colorants when the toner particles are admixed with selected carriers in, for example, two component development systems. The triboelectric properties of toners are thereby controlled or substantially dominated by the charging effects of the outer overcoating material. Accordingly for two component development, similar or substantially similar equilibrium triboelectric characteristics can be achieved with the toners of the present invention regardless of the nature of the colorants present in the toners when the same carriers are utilized. For single component development where triboelectric charging is generally accomplished by a frictional charging blade, similar equilibrium triboelectric charge levels can also be obtained under identical conditions with different colored toners of the present invention. Effective containment of core components enabled by the outer overcoating layer also inhibits the leaching of core components, thereby eliminating or substantially reducing the problem of toner blocking or agglomeration in toners wherein core resins of low glass transition temperatures are utilized. Another advantage of addition polymer overcoating relates to its ability to reduce or minimize the moisture sensitivity of the toner particles. In addition, since both the shell and the overcoating layer of the toner compositions of the present invention are relatively thin in nature, their presence would not adversely affect or would minimize the toner's fusing characteristics.
In known color reprography, such as in full color or highlight color processes, colored toners with a wide variety of colors including black are usually employed. For two component development, it is highly desirable that the triboelectric properties of different colored toners can be desirably controlled, thereby permitting them to attain similar equilibrium triboelectric charging levels when utilized against the same carriers. This is especially useful for custom colored toner packages as custom colored toners can now be generated by simple physical blending of the primary colored toners. Another important issue for two component development is the rate of charging of fresh, substantially uncharged toners to the equilibrium charge levels when they are added to the toner depleted development housing. A fast rate of charging of fresh toners is important in ensuring proper image development, particularly for high speed reprographic systems.
Color pigments or dyes have a dominant effect on the triboelectric charging behavior of toners as these colorants are often present at or close to the surface of the toner, and are therefore exposed to the environment. As a consequence, when the toner particles are admixed with carriers, the interactions of the exposed pigments of the toners with the carrier particles can essentially dominate the charging behavior of the toner. This also occurs for a number of prior art encapsulated toners where the color pigment particles are not completely encapsulated within the toner shell. Toners with identical components and different colorants often exhibit different charging behavior, sometimes even to the extent of achieving triboelectric charges of opposite polarity. To overcome this difficulty, it is usually necessary to utilize different charge control additives for different colorants or to exchange the toner with charge control additives so as to nullify or overcome the different charging effects of different colorants, and to exert a dominating influence on the charging characteristics of the toners. The toners of the present invention eliminate or substantially overcome this difficulty by completely encapsulating the colorants within the toner with an outer overcoating layer. As a consequence, the need to rely on different or high levels of charge control additives for different colored toners for achieving similar triboelectric charging levels is eliminated or substantially avoided. Other advantages associated with the encapsulated toner compositions, and economical processes of the present invention include, for example, rapid triboelectric charging rates, smaller toner size and narrow size distribution for high resolution images, excellent toner color mixing properties and image color fidelity, high image projection efficiency enabling their use on transparent substrates, lower fusing temperatures, acceptable powder flow, and nonblocking and nonagglomerating characteristics. The toner compositions of the present invention can be selected for a variety of known imaging processes including electrophotographic and ionographic processes. More specifically, the toner compositions of the present invention can be selected for electrophotographic processes, especially color processes, wherein image fixing is accomplished by heat fusion.
The toner compositions of the present invention can in one specific embodiment be generated by a direct preparative process involving a physical microencapsulation via adsorption and precipitation of polymeric surfactant, such as a cellulose surfactant, an optional shell-forming interfacial polycondensation, a core resin-forming free radical polymerization, and an overcoating free radical polymerization. In an embodiment, the combined thickness of the dispersant layer and the shell is less than about 0.5 micron, and preferably less than 0.1 micron, while the thickness of the overcoating is less than about 2 microns, and preferably less than about 0.5 micron. A thin shell and a thin overcoating could ensure that the fused image would not significantly scatter incident lights even when the refractive indices of the shell and overcoating materials do not properly match with those of the core components, which light scattering may dramatically reduce the image's projection efficiency. It is also believed that a thin shell and thin overcoating, particularly those with a thickness of substantially less than 0.5 micron but greater than zero microns, would not significantly interfere with the fusing properties of the core components, which constitute the bulk of the toner composition. One embodiment of the present invention is, therefore, directed to a simple and economical preparative process for colored encapsulated toner compositions comprised of a core comprised of a polymer resin or resins and colorants, encapsulated within a thin cellulose layer and an optional thin condensation polymer shell, and overcoated thereon a thin addition polymer overcoating. The preparative process involves an initial formation of stabilized microdroplet suspension, an optional interfacial polycondensation for the formation of condensation polymer shell, a physical microencapsulation via adsorption and precipitation of cellulose surfactant for the formation of cellulose shell, a core resin formation via free radical polymerization of addition monomers, and finally an overcoating step via free radical polymerization. The process in embodiments permits the generation of small toner size particles of less than about 10 microns in volume average particle diameter, and generally in the range of from about 2 to about 8 microns, with a narrow size distribution of less than 1.4, and in many instances, equal to or less than 1.3, without effecting the energy intensive known pulverization and particle size classification techniques. Furthermore, the process of the present invention also enables the generation of colored toner particles with controlled triboelectric properties when these particles are utilized in conjunction with selected carrier particles for two component development. The controlled charging characteristics of toner particles also render their use in single component development highly desirable since the same or substantially similar triboelectric charging levels can be readily accomplished using a frictional charging mechanism regardless of the colorants present in the toners. The aforementioned toners prepared in accordance with the process of the present invention are useful for permitting the development of images in reprographic imaging systems, inclusive of electrophotographic and ionographic imaging processes wherein the image is fused by heat, photochemical energy, pressure, or a combination of these fusing techniques.
Encapsulated toners and processes are known. For example, both U.S. Pat. No. 4,626,489 and British Patent 1,538,787 disclose similar processes for colored encapsulated toners wherein both the core resin and shell materials are prepared by suspension polymerization techniques. U.S. Pat. No. 4,565,764 discloses a colored microcapsule toner comprised of a colored core encapsulated by two resin shells with the inner shell having an affinity for both the core and the outer shell materials; U.S. Pat. No. 4,254,201 illustrates the use of pressure sensitive toner clusters or aggregates with each granule of the cluster or aggregate being comprised of a pressure sensitive adhesive substance encapsulated by coating film. Color pigment particles or magnetic particles can be present on the surfaces of the encapsulated granules to impart the desired color to the toners. Also, U.S. Pat. No. 4,727,011 discloses a process for preparing encapsulated toners which involves a shell forming interfacial polycondensation and a core binder forming free radical polymerization, and further U.S. Pat. No. 4,708,924 discloses the use of a mixture of two polymers, one having a glass transition temperature in the range of -90.degree. C. to 5.degree. C., and the other having a softening temperature in the range of 25.degree. C. to 180.degree. C., as the core binders for a pressure fixable encapsulated toner. Other prior art, all U.S. patents, are summarized below: U.S. Pat. No. 4,339,518, which relates to a process of electrostatic printing with fluorinated polymer toner additives where suitable materials for the dielectric toner are thermoplastic silicone resins and fluorine containing resins having low surface energy, reference column 4, beginning at line 10, note for example the disclosure in column 4, line 16, through column 6; U.S. Pat. No. 4,016,099, which discloses methods of forming encapsulated toner particles and wherein there are selected organic polymers including homopolymers and copolymers such as vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, and the like, see column 6, beginning at line 3, wherein there can be selected as the core materials polyolefins, polytetrafluoroethylene, polyethylene oxide and the like, see column 3, beginning at around line 18; U.S. Pat. No. 4,265,994 directed to pressure fixable capsule toners with polyolefins, such as polytetrafluoroethylene, see for example column 3, beginning at line 15; U.S. Pat. No. 4,497,885, which discloses a pressure fixable microcapsule toner comprising a pressure fixable component, a magnetic material, and other optional components, and wherein the core material can contain a soft material typical examples of which include polyvinylidenefluoride, polybutadiene, and the like, see column 3, beginning at line 10; U.S. Pat. No. 4,520,091 discloses an encapsulated toner with a core which comprises a colorant, a dissolving solvent, a nondissolving liquid and a polymer, and may include additives such as fluorine containing resin, see column 10, beginning at line 27; U.S. Pat. No. 4,590,142 relating to capsule toners wherein additives such as polytetrafluoroethylenes are selected as lubricating components, see column 5, beginning at line 52; U.S. Pat. Nos. 4,599,289 and 4,803,144.
With further specific reference to the prior art, there are disclosed in U.S. Pat. No. 4,307,169 microcapsular electrostatic marking particles containing a pressure fixable core, and an encapsulating substance comprised of a pressure rupturable shell, wherein the shell is formed by an interfacial polymerization. One shell prepared in accordance with the teachings of this patent is a polyamide obtained by interfacial polymerization. Furthermore, there are disclosed in U.S. Pat. No. 4,407,922 pressure sensitive toner compositions comprised of a blend of two immiscible polymers selected from the group consisting of certain polymers as a hard component, and polyoctyldecylvinylether-co-maleic anhydride as a soft component. Interfacial polymerization processes are also selected for the preparation of the toners of this patent. Also, there are disclosed in the prior art encapsulated toner compositions containing costly pigments and dyes, reference for example the color photocapsule toners of U.S. Pat. Nos. 4,399,209; 4,482,624; 4,483,912 and 4,397,483.
In a patentability search report, there were recited the following U.S. patents: U.S. Pat. No. 4,576,890 which discloses a process for the preparation of encapsulated toners comprising a stage of forming shells around microdroplets of a core material containing colorant dispersed in an aqueous medium to produce microcapsules therein and the stage of separating the microcapsules from the aqueous solution and wherein methylcellulose is employed for stabilizing the microdroplets of the core material in the aqueous solution, reference the Abstract of the Disclosure for example; U.S. Pat. No. 4,565,764 which discloses a microcapsule toner having a colored core material coated successively with a specific first resin wall and a second resin wall, reference the Abstract of the Disclosure, and note the disclosure in column 5 wherein the first resin wall is indicated as being obtained by reacting an olefinic carboxylic acid chloride with a core material in the presence of an acid eliminating agent, see column 5, beginning at line 30, and continuing on to column 6; also note column 7 wherein examples of the second wall are provided, and wherein examples include with respect to the first wall polyvinylalcohol, resins known in the art such as homopolymers or copolymers of monomers such as styrene or its derivatives; and as of collateral interest U.S. Pat. No. 4,524,199.
The disclosures of all the U.S. patents and other patent documents mentioned herein are totally incorporated herein by reference.
Illustrated in U.S. Pat. No. 4,758,506, the disclosure of which is totally incorporated herein by reference, are single component pressure fixable toner compositions, wherein the shell selected can be prepared by an interfacial polymerization process. A similar teaching is present in application U.S. Ser. No. 718,676 (now abandoned), the disclosure of which is totally incorporated herein by reference. In the aforementioned application, the core can be comprised of magnetite and a polyisobutylene of a specific molecular weight encapsulated in a polymeric shell material generated by an interfacial polymerization process. A number of other copending applications illustrate various encapsulated toner compositions including, for example, U.S. Pat. No. 5,043,240, U.S. Pat. No. 5,035,970, U.S. Pat. No. 5,037,716, U.S. Ser. No. 516,864, U.S. Pat. No. 5,045,428, U.S. Pat. No. 5,077,167, U.S. Ser. No. 456,278, U.S. Pat. No. 5,114,819, U.S. Pat. No. 5,082,757, U.S. Ser. No. 617,222, U.S. Pat. No. 5,023,159 and U.S. Pat. No. 5,013,630, the disclosures of each of the aforementioned copending applications being totally incorporated herein by reference.
A number of the prior art encapsulated toner compositions, in particular colored toner compositions, suffer from a number of deficiencies as indicated herein. For example, these toner compositions may not have the desirable fusing properties such as being able to be fused at reasonably low temperature of, for example, less than 170.degree. C.; they generally possess very low image projection efficiency either because of a significant difference in the refractive indices of the shell and core components or because of a poor colorant dispersion within the core; they usually require different or excessive amounts of charge control agents for different colored toners; and their rates of triboelectric charging are poor. In addition, some prior art colored encapsulated toners cannot be obtained in smaller toner size of, for example, less than 7 or 8 microns with a narrow size distribution of, for example, less than about 1.35 in a cost effective manner. Also, toner blocking or agglomeration may be a problem with several of the prior art encapsulated toners because of the porosity of the shell structure, especially when they are exposed to conditions of elevated temperatures. Further, some of the prior art colored encapsulated toners are comprised of colored pigment particles that may not completely be encapsulated by the shell, and the triboelectric charging effects of such pigments are therefore not fully passivated, and this would adversely affect and degrade the toner triboelectric characteristics, thereby causing image quality to deteriorate. These and other disadvantages are eliminated or substantially eliminated with the process and toner compositions of the present invention. More specifically, thus with the encapsulated toners of the present invention, the toner properties can in many instances be tailored to certain specifications. Specifically, with the toners of the present invention in embodiments complete or substantial passivation of the triboelectric charging effects of the colorants is accomplished, and smaller toner particle size with narrow size distribution can be achieved with the process of the present invention without conventional classification techniques. In addition, excellent image projection efficiency can be obtained with the toners of the present invention in embodiments since both the shell and the overcoating layer are relatively thin in nature. Also, the toners of the present invention do not block or agglomerate over a long period of time in embodiments.